Engine issues?

Briar Rabbit

Briar Rabbit

Line Up and Wait
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Albion, Nebraska
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Rob
A friend has a 172 with a Signature O-300, 500 hours SMOH and millennium cylinders. He started having engine performance issues so had it borescoped and found a white coating on the top of the pistons and in the top of the cylinders. He thinks there was fuel contamination from the tanker that delivered the fuel to the airport, maybe a diesel fuel additive that did not get adequately cleaned from the tanker truck. The cylinders were all pulled and the pistons, rings and cylinders thoroughly cleaned. He spent $6,000 on it. After reassembly and with about 12 more hours it seemed to perform at less than optimum so he had it borescoped again. The exhaust valves in all 6 cylinders have a green color on the inside of the cylinder(piston side). The A&P thinks this abnormal color means the valves are burned and needs to be torn down again.

He would like additional opinions of the green color of the exhaust valves? And what could have caused these issues? What should be his next steps?
 
Some one is feeding it something bad.
where is the fuel coming from ?
 
Tom-D said:
Some one is feeding it something bad.
where is the fuel coming from ?
Click to expand...
Phillips South Dakota is where the original questionable fuel was purchased prior to tear down. After that probably somewhere in Central Nebraska but I am not sure?
 
White coating sounds like mogas. $6k to address it sounds like a ripoff.
 
Briar Rabbit said:
Phillips South Dakota is where the original questionable fuel was purchased prior to tear down. After that probably somewhere in Central Nebraska but I am not sure?
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additives. ? oil ?
 
two places bad fuel?? not likely
 
what was used for pre-lub for assembly
 
Briar Rabbit said:
Is the green tint on the exhaust valves symptomatic of burnt exhaust valves?
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no.. probably not.

do this,
change oil / filter, clean spark plugs, remove all fuel, replace.

run a 100 hour run.
see what we get.
 
remember diesel fuel will run black, not white.
 
Briar Rabbit said:
...He thinks there was fuel contamination from the tanker that delivered the fuel to the airport, maybe a diesel fuel additive that did not get adequately cleaned from the tanker truck...
Click to expand...

I understood because of the lead in avgas the tankers have to be single product and cannot ever be reassigned to haul any other fuel. It's rare to see one that hauled other product come into avgas service as it's a one-way trip.
 
Agree with above. Tank trailers hauling 100LL are dedicated to that product and the drivers are usually dedicated to 100LL delivery.

Specific gravity of the fuel is checked both when it is loaded at the fuel farms and again before it is delivered in the FBO’s tanks.
 
Mike Busch video on borescope indicates that a green tint on exhaust valves can mean it is burnt and about to fail... whole video is very informative 41:20 is the spot. I’m not a mechanic but...
 
I always caveat my engine comments with the fact that I know automotive engines more than aviation engines, sometimes there is cross-over, sometimes not.

White to me means super lean. Valves may have been damaged previously and since they were “cleaned” and then run, now exhibit the green supposedly indicative of a burned valve. The cleanings addressed the symptom, not the cause.

I don’t know shinola.
 
The white—-Sounds to me like the normal crusty-looking crap that you find in low compression engines running 100LL...


Post pictures
 
Last edited:
bnt83 said:
The white—-Sounds to me like the normal crusty-looking crap that you find in low compression engines running 100LL...


Post pictures
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Wouldn't that a kick,, running great, and some one doesn't know what they are looking at
 
OP, what performance issues is the engine having apart from a perceived wrong internal coloring?
 
I talked with my friend this morning about his issues with the Continental O-300 and hopefully will be able to answer the questions above. His initial issue started with what seemed like a rough engine - more pronounced during reductions in power when approaching airports. So he asked his A&P to check the spark plugs. When they pulled the plugs they were covered with a white chalky coating, excessive coating. So the A&P borescoped the engine and it was on the pistons and upper cylinder areas. He cleaned the plugs and then they added Marvel Mystery oil to both the fuel and the oil sump hoping it would remove whatever the white coating was. He then flew it for another 6 hours and borescoped it again, it did not remove the contaminant. So the A&P pulled the jugs, the coating on the top of the pistons and head area was approximately 1/32" thick. They shined up the affected area and reassembled it. A&P suggested trying to operate it at a leaner mixture. Previously he had adjusted for max EGT then richened 100 degrees. So now he leaned for max EGT and richened 50 degrees instead. He flew it for 5 or 6 hours and it seemed to be normal but then during additional flights started noticing some engine roughness again, less than 10 hours since reassembly. So they again borescoped it - no white chalky substance but found the green tint on the exhaust valves. This green tint is in the center of all 6 valves and seems to be symmetrical in appearance. It does not sound like it is on one side of the valves as you might expect with a burnt valve failing. He does have an ancient CHT and said his CHT (one cylinder only) was running about 200 degrees - so my thought this is Celsius or else his CHT is not working right? 200 degrees Celsius is 392F and on the high side! He is going to check his CHT and let me know if it is Celsius or Fahrenheit? His CHT is about 60 years old.

All of the fuel was replaced in the engine prior to these last flights. They did notice what might have been an unusual odor not typically associated with 100LL. They did take a fuel sample from the Phillips SD airport and have sent it to a lab for analysis but the results have not been returned. It has been about two weeks since they sent it.

Suggestions?
 
I am not a mechanic (of aircraft) but I thought 50 degrees rich of peak was the absolute worst place to run one of these engines.
 
jsstevens said:
I am not a mechanic (of aircraft) but I thought 50 degrees rich of peak was the absolute worst place to run one of these engines.
Click to expand...

Depends on altitude (or HP %), below 65%...you can do whatever you want


Tom
 
FlySince9 said:
Mike Busch video on borescope indicates that a green tint on exhaust valves can mean it is burnt and about to fail... whole video is very informative 41:20 is the spot. I’m not a mechanic but...
Click to expand...

Thank you! I watched the whole thing, I learned a lot!
 
I wouldn't be worried about a 1/32" thick white stuff. If the plugs were dirty, clean them.

Based on the above, it sounds like the mechanic isn't using very good diagnostic techniques. An engine roughness may be caused by cleaning plugs, but it won't be caused by some junk on the faces of the pistons. An ignition issue sounds a lot more possible, if not some sort of carb icing. Bad fuel, maybe, but if I've ever gotten bad 100LL it's never been bad enough to notice. It is pretty tightly controlled.

As for the burned valves, green would typically be an indication of a burned/burning valve. Here's a very burned valve that was making 0/80 compression:

23517705_851657145383_8440225832803128740_n.jpg


But a lot of times borescope cameras are hard to really get exact colors on and sometimes you can see a green hue or tint when it's not actually a burned valve. This should get followed up with a compression check to see what the values are and if the exhaust valves are leaking. A truly burned valve will be very obvious on a compression check in terms of air leaking from the exhaust valve and very low compression.
 
jsstevens said:
I am not a mechanic (of aircraft) but I thought 50 degrees rich of peak was the absolute worst place to run one of these engines.
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Remember these are 0-300-A-D, leaning too much will simply run rough before they hurt themselves. rich mixtures will harm the engine.
 
Tom-D said:
Remember these are 0-300-A-D, leaning too much will simply run rough before they hurt themselves. rich mixtures will harm the engine.
Click to expand...

So the "red box" is applicable to higher compression engines then? I'm just trying to understand bits from different places.
 
jsstevens said:
So the "red box" is applicable to higher compression engines then? I'm just trying to understand bits from different places.
Click to expand...

Pretty much the red box is a graphical depiction of all mixture/HP combinations in “any”
engine that bring CHTs into the 350 degree range and above. at around 60% power the redbox disappears and any mixture is acceptable but not if the engine is complaining with roughness.
 

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jsstevens said:
So the "red box" is applicable to higher compression engines then? I'm just trying to understand bits from different places.
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we can use injected engine this way but Can't use a carb engine that way. 1 or more cylinder will run lean, and the others will run rich cylinder will be in the red box. so the 0-300 must run way rich.
 
So my O-200 can’t use the “lean till rough and back off just to smooth” method? Is that not on the lean side of peak? O-200 being the 4 cyl version of the O-300. I have never heard of anything else that was acceptable for the Cessna 150 without any other gauges.
 
Matthew Rogers said:
lean till rough and back off just to smooth” method? Is that not on the lean side of peak
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No. That's rich of peak. Peak <> rough

You lean it out until it gets rough, the roughness is caused by the different cylinders having different mixtures since the carburetor is not perfect.. with good injectors you can get remarkably lean until you start noticing reductions in power without the engine running rough
 
Tom-D said:
the 0-300 must run way rich.
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I’m not sure I believe this yet. My POH indicates that 25 LOP is acceptable but I’ve noticed that I start to get some roughness even before reaching peak EGT. But with only one EGT gauge it’s impossible to know what is really going on. I’m looking forward to picking up my airplane with its newly installed EDM830 so I can figure out where the sweet spot really is located. Just because the engine is carbureted doesn’t necessarily mean LOP is automatically impossible. But, I’ll soon know for sure.
 
Running to lean, on one cylinder will still leave 1 or more on the rich side.
#2 cylinder will be the leanest on the 0-300.

It's the nature of the beast.
we would be required to have calibrated injectors to have each cylinder run the same. (the 0-300 runs a carb)
 
FlySince9 said:
I’m not sure I believe this yet. My POH indicates that 25 LOP is acceptable but I’ve noticed that I start to get some roughness even before reaching peak EGT. But with only one EGT gauge it’s impossible to know what is really going on. I’m looking forward to picking up my airplane with its newly installed EDM830 so I can figure out where the sweet spot really is located. Just because the engine is carbureted doesn’t necessarily mean LOP is automatically impossible. But, I’ll soon know for sure.
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when was the instrument last calibrated ?
 
Tom-D said:
when was the instrument last calibrated ?
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The EGT? Probably 1965...LOL. It wasn’t working at all when I bought the plane last year. Local shop got it working a few months ago...

didn’t know those were calibrated... it doesn’t have temperature markings.
 
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